Abstract: An apparatus for the measurement in realtime of the transfer function in both magnitude and phase of an unknown system including apparatus for generating a pseudo-random binary sequence as a test signal, means to match the timing of this sequence and of the sampling process so that the discrete Fourier transform (DFT) of the test signal and the response signal obtained from the unknown system can be computed, means such that this operation can be repeated successively in precisely the same manner without significant time (phase) jitter, means such that the magnitude function obtained from any single transformation can be computed and displayed, and means such that the difference between the phase functions obtained from two distinct DFT's can be computed and displayed.